1. Field of the Invention
The present invention relates to an oxygen electrode, a biosensor, and processes for manufacturing the same. The oxygen electrode is used to determine the concentration of gaseous oxygen dissolved in a liquid and the oxygen electrode may be used to make a biosensor.
2. Description of the Related Art
An oxygen electrode is composed of a working electrode (cathode) and a counter electrode (anode) in an electrolyte such as potassium chloride. The dissolved oxygen and water react with electrons on the working electrode (cathode) to generate hydroxide ions which result in electric current between the counter electrode or anode and the working electrode or cathode in an amount in linear relation to the concentration of the dissolved oxygen. With the oxygen electrode, the electric current can be measured to estimate the concentration of the dissolved oxygen. The oxygen electrode may optionally further comprise a reference electrode.
A biosensor for determining, for example, a carbon dioxide gas concentration, a concentration of a glucose or alcohol, or the like, may be realized by combining the oxygen electrode with autotrophic microorganisms, for example, one taking carbon dioxide gas as the nutrient, or various enzymes and by measuring the oxygen concentration with the oxygen electrode since the consumption of the carbon dioxide gas results in the variation of the oxygen concentration.
The oxygen electrode can be used in various industrial fields including the measurement of the biochemical oxygen demand (BOD) in water, the measurement of the dissolved oxygen as a parameter of the progress of a fermentation of glucose or alcohol in a fermentation tank, and others.
An example of the above biosensor is a glucose concentration sensor using glucose oxidase which oxydizes glucose and transforms it into gluconolactone. The glucose concentration sensor estimates the glucose concentration by using as a parameter the decrease of the dissolved oxygen concentration resulted from the above oxidation reaction. Such glucose concentration sensors are utilized in fermentation industries and the determination of blood sugar value, etc.
A typical oxygen electrode is illustrated with reference to FIG. 1, in which 1 denotes a working electrode (cathode), 2 a counter electrode (anode), 3 an electrolyte, and 4 a gas permeable membrane. The oxygen dissolved in the electrolyte through the gas permeable membrane 4 and water react with electrons on the working electrode (cathode) to form hydroxide ions OH-- (i.e., O.sub.2 +2H.sub.2 O+4e-=4OH-) and cause an electric current in relation to the amount of the dissolved oxygen so that the oxygen concentration can be estimated from the electric current detected.
This oxygen electrode is however large in size and is difficult to be manufactured in mass production. The inventors have developed and disclosed a small oxygen electrode in which recesses are formed on a surface of a silicon substrate by anisotropic etching and the recesses are separated with each other, filled with an electrolyte, and covered with an oxygen permeable membrane (Japanese Unexamined Patent publication No. 63-238548; Japanese Patent Application No. 62-71739). The inventors have also developed and disclosed an improved small oxygen electrode in which anodic bonding is utilized to ease commercial manufacturing of the oxygen electrode (Japanese Unexamined Patent publication No. 04-125462; Japanese Patent Application No. 02-243849).
Nevertheless the oxygen electrodes and biosensors in the prior art are still expensive and cannot be used as disposable products. On the other hand, strong demand for such oxygen electrodes and biosensors exists and lowering of the price thereof is required.